An infrared system may operate in an environment where the system views the scene through a window while flying through the atmosphere. The window will heat throughout the mission resulting in a dynamic background. The camera is calibrated with a two-point fit to a response measurement for each pixel. There is a residual gain error in the calibration due to a calibration error, there are errors due to non-linearity of the pixel response and there is instability in the response over the storage life of the seeker. These gain correction errors when coupled to a highly dynamic background which shift the operating point (background flux level) of the camera can result in significant degradation of uniformity (noise) impacting mission performance.
Attempts to perform automatic offset correction based on the scene using either sensed or induced motion (dither) are inherently limited in the presence of objects in the field of view. These approaches tend to assume that the displacement of the object space image allows an algorithm to separate the object space from the camera fixed pattern noise that does not shift. The problem with these approaches is that the object space image will change from one frame to the next as it moves with respect to the camera system. Also, the finite spatial sampling of the camera assures that there will be substantial differences in the count distribution of an object image due to displacement of the object relative to the sampling grid.
The prior art includes a Jones calibration source which is used to calibrate cameras. The Jones calibration source is a small source which is placed near an entrance pupil of a camera system to uniformly flood a focal plane array and is described in The Infrared Handbook by William L. Wolfe and George J. Zissis (revised edition 1985, prepared by Infrared Information and Analysis (IRIA) Center, Environmental Research Institute of Michigan for the Office of Naval Research, Department of the Navy, Washington, D.C.). The focal plane array is a charge-coupled device which has an output which is in electronic form.